Endoglin (ENG, chromosome 9q34), Activin A receptor type II-like 1 (ACVRL1/ACVRL1/ALK1, chromosome 12q13), and MADH4/SMAD4 (chromosome 18q21) mutations cause HHT1 (OMIM 187300), HHT2 (OMIM 600376), and the combined Juvenile Polyposis/HHT (JP/HHT) syndrome (OMIM 175050), respectively [5–7]. The gene discussed is SMAD4; the disease is hereditary hemorrhagic telangiectasia.